A thermistor is a device in which the resistance or resistivity changes in a predictable and repeatable manner when exposed to a specified range of temperatures. A thermistor can be used to measure a set temperature or to monitor temperature variation. The potential usefulness of a particular material for producing a thermistor is judged on a number of factors including the rate of change in the material's electrical properties with a change in temperature; the consistency or linearity of that change throughout a given range of temperatures; and the ability of the material to be used over a wide range of temperatures. Other factors include ruggedness, ease of manufacture, cost, useful life and the like. A thermistor made from a particular ceramic material may be ideal when used at temperatures of between 800.degree. C. and 900.degree. C. However, despite the obvious value of such a thermistor, its relatively narrow temperature window would limit its overall utility to applications which constantly required the use of temperatures between 800 and 900.degree. C. Accordingly, it has always been desirable to discover materials which can be used to produce thermistors having as wide a useful temperature range possible.
Thermistors based on lanthanum have been used in the past because of their relatively broad range of applicability. Lanthanum chromium thermistors, for example, can be used at temperatures as low as -50.degree. C. and as high as about 900.degree. C. Unfortunately, these LaCrO.sub.3 based thermistors must be glazed with a glass or glass-like coating because LaCrO.sub.3 undergoes a structural transition at 270.degree. C. which causes a resistance change. This transition is prevented by glassing or glazing. Unfortunately, most of the glazes are only useful at temperatures up to about 900.degree. C. Above about 900.degree. C., they melt thereby exposing the lanthanum based thermistor to the undesirable structural transition.
Iwaya et al., U.S. Pat. Nos. 5,476,822, 5,610,111 and 5,637,543 each disclose thermistors made from various ceramic compositions. The '543 patent describes a ceramic composition for a thermistor represented by the formula (YCrO.sub.3).sub.1-x (YAl.sub.3).sub.x where 0.8 &gt;X&gt;0. The '822 patent contains similar claims although much broader in scope. Again, however, an aluminum oxide based phase is required. The '111 patent claims a ceramic composition for a thermistor represented by the formula: (Y.sub.1-x Sr.sub.x) (Cr.sub.1-y-x Fe.sub.y Ti.sub.z)O.sub.3 where X, Y and Z are specifically defined.
All three of these patents describe the resulting ceramic composition as being useful over a range of from room or ambient temperature up to about 1100.degree. C. However, a review of their complete disclosures suggest applicability only between about 300.degree. C. and 1100.degree. C. Indeed, a review of table 1 of the '822 patent demonstrates that the Beta values of the disclosed materials was higher at lower temperatures than at higher temperatures. Moreover, the aluminum oxide phase is a high resistance insulator type material. As a result, these formulations are not generally suitable for use at lower temperatures.